1. Field of the Invention
The present invention generally relates to track address patterns and, more particularly, is directed to a track address pattern for use with a disk memory apparatus which positions a head on a track having a target address according, for example, to the sector servo system.
2. Description of the Prior Art
As shown in FIG. 1, a magnetic disk MD used as a record medium by the magnetic disk apparatus, such as a so-called hard disk apparatus, a floppy disk apparatus and so on is divided along the radial direction (direction shown by an arrow R in FIG. 1) thereof to provide a plurality of tracks TR1, TR2, . . . , TRN. Also, the magnetic disk MD is divided along the circumferential direction (direction shown by an arrow .phi.) thereof to provide a plurality of sectors 2A, 2B, . . . , whereby the recording or reproducing region can be specified by the track number (track address) and the sector number.
As a control method for positioning a recording/reproducing head on the magnetic disk along its radial direction so that the recording/reproducing head is maintained on the central axis of a track having a predetermined track address, a so-called sector servo system is known, in which normal data and positioning servo data are recorded on the recording surface of the magnetic disk in a time division fashion. More specifically, in the example of FIG. 1, areas between the sectors 2A, 2B, . . . are utilized as servo zones 1A, 1B, 1C, . . . and a track address for coarse positioning, a burst pattern for fine positioning or the like are recorded on these servo zones 1A, 1B, 1C, . . . , respectively.
A gray code is frequently utilized as the track address because only one bit is different in adjacent codes of the gray code so that, even when the head scans two tracks at a time, a value reproduced by the head can indicate the address of any -of the two tracks. Further, even when the head scans three tracks at a time in the high speed seek mode, a value reproduced by the head indicates the address of the track distant by two tracks at best from the center track. When the gray code is utilized as the track address of the magnetic disk, a code (source bit) must be converted into a channel bit by the modulation so as to satisfy the following two conditions.
Condition 1) When, for example, the magnetic flux inverted state is made corresponding with the high "1" level and the magnetic flux non-inverted state is made corresponding with the low "0" level in order to enable so-called self-clock to be made to form a clock from the reproduced data, the number (run length) of consecutive "0"s must be set to be less than a predetermined value in order to limit the recording length of the magnetic flux non-inverted state; and
Condition 2) When the magnetic flux is inverted in the adjacent tracks at the same position along the circumferential direction, the magnetic flux must be inverted in the same direction in order to avoid the interference of magnetic flux and also to accurately detect the change of magnetic flux when the head scans these tracks.
As a modulation method for satisfying these conditions 1) and 2), a phase modulation (PM), a dibit conversion and a frequency modulation (FM) are known. In the phase modulation, as shown in FIG. 2A, low "0" level and high "1" level of the source bits are converted into two channel bits "01" and "10", respectively. In this case, the channel bit "0" corresponds to the state that the magnetic flux in one bit cell Ts on the recording track is not inverted while the channel bit "1" corresponds to the state the magnetic flux in one bit cell Ts on the recording track is inverted. Thus, two bit cells Ts constitute one cell on the recording track corresponding to one bit of the source code. Accordingly, the Hamming distance, which represents the number of different bits between address data recorded on the adjacent tracks, constantly becomes two channel bits to thereby satisfy the above conditions 1) and 2). Alternatively, the source bits "0" and "1" may be rendered to correspond to two channel bits "10" and "01", respectively. In this phase modulation, a code rate which is a ratio between the number of source bits and the number of channel bits, that is, a ratio of original data amount to data amount on the magnetic disk becomes 0.5.
In the dibit conversion, the source bits "0" and "1" are converted into two channel bits "00" and "11", respectively, as shown in FIG. 2B. However, in this dibit conversion, additional redundant data is required to satisfy the condition 2) (the run-length condition), making the code rate become smaller than 0.5.
In the frequency modulation, the source bits "0" and "1" are converted into two channel bits "10" and "11", respectively, as shown in FIG. 2C. However, in this frequency modulation, the magnetic flux inverted directions of the adjacent tracks are reversed as, for example, at a position P1 in FIG. 2C, so that the condition 2) can not be satisfied. To satisfy the condition 2), additional data is required, which unavoidably makes the code rate become smaller than 0.5.
As described above, of the conventional modulation methods, the phase modulation has the highest code rate and the conventional servo pattern utilizing the phase modulation described in U.S. Pat. No. 4,032,984 will be described with reference to FIGS. 3 and 4.
FIG. 3 shows the servo pattern, and as shown in FIG. 3, many tracks concentrically arranged in parallel are divided into servo tracks and data tracks in the circumferential direction, respectively. In this servo pattern of FIG. 3, magnetic flux is inverted in the respective servo tracks commonly at the synchronizing position P2 in the same direction; magnetic flux is inverted in every other tracks n, n+2, n+4, . . . , at the position P3 in the same direction; and magnetic flux is inverted in every other tracks n+1, n+3, n+5, . . . , at the position P4 in the same direction. Data at the positions P2, P3 and P4 form fine positioning servo data.
As shown in FIG. 3, cells A, B and C are provided after the position P4. Each of the cells A, B and C has two bit cells Ts having values "1" and "0" shown in FIG. 2A, and by these three cells A, B and C (six channel bits), gray codes of 3 bits 000 to 111 as track addresses (coarse positioning servo data) are phase-modulated and recorded.
FIG. 4 shows a reproduced signal reproduced by gradually moving the head from the servo track n to the servo track n+7 along the radial direction of the magnetic disk when the magnetic disk is rotated at high speed under the condition such that the gray codes 000, 001, 011, . . . , are made corresponding with the servo tracks n, n+1, n+2, . . . Since the reproduced signal is produced by differentiating the magnetic flux from a time standpoint, the reproduced signal becomes a positive or negative pulse at the magnetic flux inverted position in response to the magnetic flux inverted direction.
A servo circuit for processing the reproduced signal is supplied with the positive or negative pulse at the timing based on the synchronizing pulse at the synchronizing position P2 and generates track addresses and fine positioning data which will be described later. More specifically, when the reproducing head is located on the servo tracks n and n+1, detection codes (gray codes) 000 and 001 are detected as track addresses, and the track on which the reproducing head is located is specified by converting the detection codes into the ordinary binary codes. Further, when the reproducing head is located between the adjacent servo tracks n and n+1, the minimum digit of the detection code becomes value T of "0" or "1". Similarly, when the reproducing head is located between the two servo tracks, one bit of the detection code becomes the value T.
The reproducing head is finally positioned on the even-numbered address track as follows: After a detection head is moved to the position at which the pulse of the position P3 is large, the pulse of the position P4 is substantially zero and the detection code corresponds with the target even-numbered address, the reproducing head is moved in the direction in which the track address is increased to thereby adjust the pulses at the positions P3 and P4 to become substantially the same in level, thus the reproducing head being positioned at the boundary line between the adjacent servo tracks. Similarly, when the reproducing head is positioned to the odd-numbered address track, the detection head is moved to the position in which the pulse at the position P3 is substantially zero, the pulse at the position P4 is increased and the detection code corresponds to the target odd-numbered address so that boundary lines of the servo tracks correspond with center lines of data tracks d, d+1, d+2, respectively.
When the phase modulation is carried out as described above, the code rate becomes 0.5 and thus the track address can be recorded most efficiently. It is, moreover, advantageous that ordinary data can be recorded much more if the recorded length of the track address on the magnetic disk is reduced by making the code rate become close to 1 as much as possible.
Further, in the high speed seek mode, for example, the reproducing head obliquely scans the coarse positioning servo data areas in FIG. 4 so that, address can be reproduced with smaller error if the track address is compressed more in recording mode. Accordingly, by increasing the code rate of the track address in the recording mode, it is possible to reduce a time to seek a track having a desired address.